(a) Field of the Invention
The present invention relates to a power supply.
(b) Description of the Related Art
The switch mode power supply (SMPS) rectifies an input AC voltage to generate a DC link voltage, and converts the DC link voltage into a different level of a DC output voltage. The DC output voltage may be higher or lower than the DC link voltage. This SMPS is mainly used in electronic devices, particularly battery power supplies such as for mobile phones and laptop computers. The SMPS has a normal operation mode where it consumes a relatively large amount of power, and a standby operation mode where it consumes a relatively small amount of power.
For improving the efficiency of the SMPS in a standby operation mode, burst mode switching is used while the switching frequency is limited to a constant value. The burst mode switching means that the SMPS repeats one operation for outputting power through a switching operation during a certain time and another operation for stopping the switching operation during a certain time.
However, the switching power loss is proportional to the switching frequency and a square of a voltage in a switching transistor. Therefore, there is a limit in reducing power loss only by controlling the switching frequency in a standby operation mode.